Tech specs say Pianos needs 230Gb and HO needs 680GB, for a total of 910Gb.

I don't have USB3, so the HD that HO is shipped on will not be used. It will just sit on the shelf as the backup storage of the original.

Instead, since I have a few HD bays empty, I'll install internal HDs dedicated to these EW VSTs. But what is best in terms of performance, reliability and cost?

Option 1 - a single 1Tb SSD to hold both. Cutting it kinda close - I think SSD's, like most HDs, like a little more free space than that to operate well.

Option 2 - a dedicated 500Gb SSD for Pianos and a 1Tb SSD for HO. Probably functionally best, but super pricey. Budget is not unlimited, and I would go this way only if no other option is possible with good performance.

Option 3 - two mechanical internal 7200rpm HDs. A 500Gb for Pianos and a 1Tb for HO.

I just bought HO myself, with Solo instruments and that takes over 800GB. Yours with Pianos will weigh in at 900GB, no problem for a 1TB SSD that will format to about 940GB, SSDs don't need 20% breathing room like mechanical drives.

My suggestion is the most expensive but you'll love the speed, and that's the newer NVMe SSD's, which are faster than the older SATA SSDs by three orders of magnitude (Specs: Sequential Read Speeds up to 3500MB/s and Sequential Write Speeds up to 2100MB/s!!). Problem is it's newer technology and costs $$$. Here's the cheapest NVMe 1TB drive right now:

I tried a couple, this Silverstone works great. If $600 isn't going to happen, then a standard 1TB SSD is the way to go, I'm on that now with plans to upgrade to the setup above (I already have this setup on a 500GB version for another library, it loads so fast it's crazy).

Forget mechanical drives, I created a 6TB RAID0 set with WD Red drives on a real LSI 3Ware PCI RAID card (instead of just using my mobo) and put Composer Cloud on that and it's fast, but when I bought HO the other day, I moved HO to the SSD, and even a regular SSD smokes the RAID0. Hop up to NVMe and your friends will be asking to borrow your studio, lol.